I came up with this function to try to extract the main diagonal.
getDiag :: [[a]] -> [a] getDiag = map (head . head) . iterate (tail . map tail)
The problem is, this function doesn't work unless I have an infinite grid.
Could anyone provide me with some hints to lead me in the right direction?
I think Dan's hint is pretty good, but a hint for this *specific* part of the problem, rather than the whole thing. 'head' and 'tail' blow up on empty lists. So any kind of solution involving iterate and similar with them tends to eventually blow up on finite lists. (take 1) and (drop 1) are rather similar functions, but they simply give [] on [] instead of blowing up. Then, on a finite list, you just keep getting []s after a while, which you can trim with takeWhile (not . null). Another approach is to replace iterate with an unfoldr; an unfoldr is rather like a 'general iterate' which 'knows when to stop' : it stops when your unfolding function gives Nothing. Jules